Standards have been provided for digital versatile discs (DVDs), or digital video discs, super audio compact discs (SACDs) and so on as mediums adapted to multi-channel reproduction using two or more than two independent channels.
The provisions on the positions of multi-channel speakers in these standards are based on the ITU-R (International Telecommunications Union Radio-communication Sector) recommendation BS-775-1 “Multi-channel Stereophonic Sound System with and without Accompanying Picture”.
FIG. 1 of the accompanying drawings schematically illustrates a standard speaker arrangement for a multi-channel stereophonic sound system according to the recommendation. The illustrated speaker arrangement involves 5 channel speakers including a front left channel L, a front right channel R, a front center channel C, a surround left channel LS and a surround right channel RS as arranged relative to listener U. A so-called 5.1 channel arrangement obtained by adding a sub-woofer (SW) speaker for low frequency enhancement (LFE) as shown in FIG. 2 is also a standard arrangement.
On the other hand, an audio reproduction device for replaying such a multi-channel medium is provided with independent audio reproduction circuits and so many audio output terminals, the number of which corresponds to the maximum number of channels that the multi-channel medium has. If an optical disc stores an audio source having 5 channels or 5.1 channels, the output terminals of the reproduction device for replaying the optical disc are connected to an external amplifier having 5 or 5.1 channel input terminals and the external amplifier is connected to the speakers corresponding to the 5 channels or 5.1 channels.
Meanwhile, when measuring the acoustic characteristics of each channel in a multi-channel acoustic reproduction environment where a DVD is used as audio source, a pink noise or white noise that is called test tone is reproduced for each channel so as to be heard by the user and the inter-channel level difference, if any, is manually adjusted by a user typically by means of a remote control unit.
Some expensive high quality AV amplifiers are provided with a functional feature of automatically adjusting the level difference and the difference in the propagation distance. With such a feature, a gauging microphone is placed at a listening position and the amplifier is equipped in the inside thereof with a test generator. A test tone is reproduced on a channel by channel basis so as to be picked up by the microphone. Then, the signal representing the picked up test tone is used to compare the picked up test tone with the original test tone to measure the inter-channel level difference and the difference in the propagation distance (time) among the channels for automatic adjustment.
With any of the above described systems to be used in a multi-channel acoustic reproduction environment, a test tone needs to be reproduced for each channel.